Steady-state Separation Research Articles

Tribology of thin-film media in both flying and sliding modes

Experimental, theoretical and numerical modelling issues concerning the tribology of thin-film rigid disk media are reviewed. Special emphasis is placed on the role of carbon overcoatings in friction and wear reduction at the head-disk interface (HDI), especially with regard to ultra-low flying or quasi-contact recording. An overview is provided of wear and failure mechanisms of carbon-coated media as observed under a variety of testing conditions. As the steady-state separation between slider and disk falls below 100 nm, novel techniques will be required for the acceleration of wear and for modelling of the HDI, some aspects of which are presented. In addition, the complementary roles of experimental techniques operating in the sliding and flying regimes, and operation near the boundary between flying and sliding, are discussed.

Performance of a Six-Port Simulated Moving-Bed Pilot Plant for Vapor-Phase Adsorption Separations

Abstract A simulated moving-bed (SMB) pilot plant, characterized by two unique features, the number of ports (6 instead of 24, as in most industrial applications) and the fluid phase (vapor iather than liquid), has been built and operated. Such a plant has proven capable to achieve complete separation for a mixture of m- and p-xylene using isopropylbenzene as desorbent and KY zeolites as adsorbent. The dynamic behavior of the unit has been investigated and compared with model predictions, both in terms of approach to cyclic steady state as well as of responses to typical inputs. The role of the key design parameters on the steady-state separation performance of the unit has been analyzed cxperimentally. demonstrating the consistency with the predictions of the Equilibrium Theory.

Performance Analysis of a Continuous Rotating Annular Electrophoresis Column

Abstract A new continuous electrophoresis column is proposed which will allow for the continuous separation of industrial scale multicomponent mixtures. Computer simulated calculations predict steady-state bed temperature, separation distance, and power requirements. The proposed design compares favorably to previous designs in terms of minimizing peak bed temperature and power consumed.

The performance of cyclic and steady-state electrodialysis separation processes in the presence of iron(II) fouling

The performance of a bench-scale cyclic electrodialysis process was compared to that of a conventional process under fouling conditions. The feed solution contained 2000 ppm sodium chloride and 0, 1, and 5 ppm ferrous chloride. Each run lasted for 20 hours and the amount of deposit and its effects on cell performance, including sodium chloride separation factor and pressure drop, were measured under a variety of operating conditions. Higher separation factors were obtained with the unsteady-state process and, unlike the conventional operation, the pressure drop did not increase during a run. However, inspection of the membrane surfaces exposed to the process fluid showed that deposition was not completely prevented by the periodic reversal of the current and flow direction. A small amount of deposit was also observed in the non-flow region between each membrane-pair, which could prove fatal to the cyclic process in the long run.

Separative Diffusion in the Transient State. III. Diffusion of a Heavy Fluid in a Cone

Abstract Diffusion of a heavy fluid into a cone is approximately one-dimensional. This process can be calculated by the differential equation of diffusion with chemical reaction. Numerical computation was applied for the study of separative diffusion. Separation factors and output quantities were found for cone angles of +30, +5, and −5[ddot] and for double barrier thickness. A periodic quasi-steady-state process is proposed which yields a reasonable output with tenfold steady-state separation.

An experimental method for studying thermal diffusion in mixtures containing the adsorbable gas Rn

An experiment has been constructed for studying thermal diffusion in the binary mixture 222Rn-CO2 by means of the two-bulb method. A smaller thermodiffusional chamber has been constructed as a proportional counter to facilitate the continuous monitoring of the 222Rn concentration during the whole measurement. A difference method has been used for determining the steady-state separation factor in order to eliminate the effects of the adsorption of radon on the walls of the apparatus. The thermal diffusion factor alpha T has been measured at the temperature T=338.8K.

Sensitivity of a process to component faults

A process simulation involving pressure, energy and mass balancing, prior plant layout and detailed models for all process units, piping, valves, pumps and blowers has been used to explore the effect of various disturbances caused by partial failure of a component part on the total system. Analysis shows that maintenance of product specifications within acceptable limits is significantly sensitive to faults occuring upstream of, or within, the system reactors but much less sensitive to faults arising downstream of reactors. Simulation involves approximations (steady-state, pseudo-ideal reactions and phase separations assumed) to the real system but results obtained are considered representative. Implications regarding importance of local unit malfunctions to overall system performance and which measurements need be routinely recorded are discussed.

Unsteady laminar separation over impulsively moved cylinders

A numerical method is employed to integrate the unsteady laminar boundary layers about circular and elliptic cylinders started impulsively from rest. An upwind differencing scheme permits the integration through thin layers of reversed flow that are embedded in the boundary layer. The results are in agreement with numerical solutions of the full Navier-Stokes equations and with small time inner-outer expansion solutions. At later times, a singularity of the Goldstein type appears; Sears and Telionis defined this singularity as unsteady boundary-layer separation. For the circular cylinder, this singularity travels upstream and for large times, arrives asymptotically at the point of steady-state separation.

NMR Measurements of Liquid Crystals of Poly-γ-Benzyl-L-Glutamate in Static Electric Fields

A couple of parallel-plate electrodes have been made of Nesa glass. NMR spectra of liquid-crystalline solutions of poly-γ-benzyl-L-glutamate in static electric fields obtained using these electrodes are reliable. The signal of a solvent molecule such as dichloromethane or dibromomethane splits into a doublet owing to magnetic dipole-dipole interactions as in the case of the magnetic orientation of these solutions, indicating that the solvent molecules orient in the direction of the electric field. Dilute liquid-crystalline solutions which still do not contain isotropic portions evidently have a strong center signal showing presence of free solvent molecules. Orientation of the solvent molecules is apt to be disturbed by a weak electric current through the solution and magnitude of the steady-state separation of the doublet can not be used as a measure for the polymer (and molecular cluster) orientation.

Differential charge transport in thunderstorm clouds

Measurements of charge versus size of drops within warm thunderstorm clouds indicate that the mean specific charge per gram decreases dramatically as the size increases. Because coalescence between smaller and larger drops occurs with an approximately geometric cross section, the charge neutralization length (the distance in which the smaller drops of one charge sign could neutralize the charge of the larger drops by coalescence) becomes small in comparison with cloud dimensions and hence ensures that any effective charge separation mechanism rapidly approaches steady state. The downward, steady-state, large drop mass flux averaged over the cloud is usually less than the upward, convected, small drop mass flux. At the interior of the cloud, quasi steady-state charge separation requires an equal and opposite charge flux carried by smaller and larger drops. The two conditions are inconsistent with current drop charge versus size measurements by several orders of magnitude. Currently, the assumption of equal and opposite charge flux, is not substantiated and hence charge separation by differential ‘falling’ of larger and smaller drops within the cloud should be a small contribution to the total charge transport.

Thermal Diffusion Columns and Intermolecular Potential. III. The Noble Gases Ar and Xe

A hot-wire thermal-diffusion column is used to study steady-state separation. The relation ln Q̄0*exptl = m̄ ln Q̄0*theoret, which was extablished for Kr in earlier works in this series, is now extended to Ar and Xe in this type of column, and m̄ is shown to be a temperature independent constant in the range of our measurements. FJO theory gives a very good prediction for the optimum pressure measured experimentally in well constructed and operated columns for all temperatures of the wire (400–1100°K). Different molecular interaction parameters chosen for the theoretical calculation of p*theoret, when these are all reasonably realistic, give very similar results for p*theoret. The differences are similar to those obtained when interpreting, with the same parameters, the viscosity coefficients of the gas in the mean temperature range of the column. From these methods of comparison some conclusions can be drawn about constructional facts.

Fine Structure of Scalar Fields Mixed by Turbulence. I. Zero-Gradient Points and Minimal Gradient Surfaces

Turbulent mixing of passive scalar properties such as temperature or concentration is discussed. From physical and geometrical considerations it is concluded that the smallest scale features of a steady-state turbulent scalar distribution are primarily determined by the number and distribution of points in the fluid for which the scalar gradient vector is zero, and surfaces for which the gradient magnitude is minimal. Mechanisms for the production, destruction, and motion of such “zero gradient points” and “minimal gradient surfaces” are examined. Initially, zero gradient points must be produced from regions of uniform scalar gradient, but the vast majority result from secondary splitting due to strain induced eccentricities of the closed isoscalar surfaces surrounding maximum or minimum points. An expression for the velocity of surfaces of constant scalar value is derived and used to interpret the Obukhov-Corrsin length scale (D3/ε)14 as the minimum size eddy capable of generating a zero gradient point from a region of uniform gradient, where D is the molecular diffusivity of the scalar and ε is the mean viscous dissipation rate. A steady-state separation distance between zero gradient points of order (D/γ)12 is inferred for scalar fields of arbitrary diffusivity where γ is the root mean square rate of strain of the fluid. Previous theories such as that of Batchelor, Howells, and Townsend have assumed the local strain rate has no effect on scalar structure for large Prandtl number fields.

Operation of Thermal-Diffusion Column with Mixtures of Gases Having Equal Molecular Weights (CO2–N2O)

Steady-state separation factors of 1:1 carbon dioxide—nitrous oxide mixture at mean temperatures 363°, 401.5°, and 435.5°K and at pressures ranging from 0.30 to 0.95 atm have been measured by operating a coaxial thermal-diffusion column. It has been found that the variation of separation factor with pressure agrees qualitatively with Furry—Jones' theory for isotopic mixtures. A detailed comparison of the experimental results with the theory has been made on the Maxwellian and Lennard-Jones (12:6) models and values for the thermal-diffusion factor for this polyatomic gas mixture have been estimated.

Thermal Diffusion in Molten Binary Mixtures of Silver Nitrate with Alkali Nitrates

Thermal diffusion in the molten systems LiNO3—AgNO3, KNO3—AgNO3, RbNO3—AgNO, and CsNO3—AgNO3 was detected by means of chemical analysis. Two types of cells were used: the elementary effect was measured directly in cells with vertical temperature gradients, and the coefficient of thermal diffusion (D') was estimated from the steady-state separation obtained in convection cells (horizontal temperature gradient). A discussion of possible sources of error shows for both methods that the results might be systematically lower than corresponding to ideal conditions. In all systems AgNO3 was enriched at the cold side. There are indications that the SORET-coefficient varies with the composiiton of the mixture and the temperature. Thus for the LiNO3— AgNO3 system, depending on the conditions, the SORET coefficient ranged from 2 × 10-3 to 5.5 × 10-3 degr.-1, while it was about 1.8 × 10-3-5 degr.-1 over a wide concentration range for the KNO3—AgNO3 system, and about 1.3 × 10-3 degr.-1 over the investigated ranges for the RbNO3—AgNO3 and CsNO3—AgNO3 systems. Also isotope effects were measured with the convection cell, giving SoRET-coefficients of the order of 10-5 to 4 × 10-5 degr.-1 for 6Li—7Li as well as for 39K—-41K, which is in agreement with previous investigations.

Isotope Enrichment Factors for Nitrogen-15 in the Nitric Oxide-Nitric Acid Exchange System

Single stage enrichment factors, α, for the isotope exchange reaction of nitrogen-15 between oxides of nitrogen and solutions of nitric acid have been determined by two different methods. Single stage equilibrations resulted in values of α that decrease from 1.065 for approximately 1M HNO3 to 1.045 for 11.1M HNO3. At high acid concentrations, α decreases rapidly owing to high concentrations of NO2 and N2O4 in the gas phase. Steady-state over-all separations in an exchange column at several rates of product with-drawal gave an independent determination for the 10M HNO3 used in the preparation of highly enriched nitrogen-15. Data from the two methods result in a value for α of 1.055±0.005 for approximately 10M HNO3 at 25°C.